This invention relates in general to acoustic wave sensors and in particular to a lateral field excited acoustic wave sensor.
Piezoelectric materials, such as crystalline quartz, generate an electric field or voltage when subjected to mechanical stress and, conversely, generate mechanical stress when subjected to an electric field or voltage. Accordingly, piezoelectric materials have proven useful in many diverse technologies. Typically, electrodes are deposited upon the surface of the crystal and an AC voltage is applied to the electrodes to generate an electric field in the crystal. The electric field, in turn, generates mechanical stresses in the crystal. If the applied AC voltage is at or near the resonant frequency of the crystal, a resonant acoustic wave is excited within the crystal. At the resonant frequency, which is determined by the cut angle, thickness, length, width and mass of the crystal, the acoustic wave may propagate and resonate within the crystal with very little loss.
A measure of how narrow a band of frequencies can be passed through a particular piezoelectric crystal with minimum attenuation relative to the resonant frequency of the crystal is referred to as the Q of the crystal. The Q of the crystal, which is a function of the crystalographic orientation of the crystal, determines the specific application for the crystal. For example, very low Q crystals are capable of converting wide frequency bands of mechanical energy to electrical energy; and, conversely, wide frequency bands of electrical energy to mechanical energy. Thus, low Q materials are often used as sonic transducers in microphones or speakers because the low Q allows many tones to be produced. With a very high Q material, only a very narrow band of frequencies may be passed through the crystal. Thus high Q material is typically used in devices that require highly accurate frequency control, such as oscillators.
High Q piezoelectric materials also are used in sensors. With modern manufacturing methods, precision crystals of quartz or other similar very high Q material may be made to oscillate at a frequency that is accurate to within a few parts per million or less. During production of such quartz resonators, layers of conductive electrode material may be deposited with a precision of a few atomic layers. The resonant frequency of the resulting resonators will be sensitive to extremely small changes in the mass of the electrodes. This characteristic sensitivity of high Q piezoelectric materials to changes in mass has led to a number of diverse sensor applications. For example, a quartz resonator may be coated with a sorbent which is selective to a particular compound. The amount, or concentration, of the compound can then be determined by monitoring the change in the resonant frequency of the quartz crystal as the compound is absorbed by the sorbent since, as the compound is absorbed, the mass of the sorbent and, hence, the total mass of the vibrating structure increases. Because the addition or subtraction of mass to the piezoelectric material results in a change of the resonant frequency of the crystal, such devices are commonly referred to as a Quartz Crystal Microbalances (QCM's) and are widely used in applications where a change in mass, density or viscosity is monitored, such as in sensing applications.
Referring now to the drawings, a typical known QCM sensor is illustrated generally at 10 in FIGS. 1 and 2. The sensor 10 includes a disc shaped substrate 12 of quartz having a diameter of approximately 25 mm. The standard crystallographic orientation used is an AT-cut since it is a temperature stable orientation in which only a Transverse Shear Mode (TSM) acoustic wave can be excited. Other orientations in quartz in which only a TSM acoustic wave can be excited also may be utilized. FIG. 1 shows the reference surface 14 of the substrate while FIG. 2 shows the sensing surface 16 of the substrate 12 that is opposite from the reference surface 14. A disc shaped reference electrode 18 formed from an electrically conducting material and having a diameter of approximately 6 mm is deposited upon the center of the reference surface 14. The electrode 18 is formed from an electrically conductive metal. The reference electrode 18 is connected by a first strip 20 of conductive material to an arcuate reference electrode tap 22. The reference electrode tap 22 allows electrical connection to an external sensing circuitry (not shown). The electrical connection is illustrated by a wire lead 24; however, the lead 24 is intended to be exemplary and other types of conventional electrical connections may be utilized.
As shown in FIG. 2, a disc shaped sensing electrode 26 formed from an electrically conductive metal and having a diameter of approximately 13 mm is deposited upon the center of the sensing surface 16. A second strip of conductive material 28 extends from the sensing electrode 26 to the edge of the sensing surface 16, transversely across the side of the substrate 12 and onto the reference surface 14, as shown in FIG. 1, where it ends in an arcuate sensing electrode tap 30. Similar to the reference electrode tap 22, the sensing electrode tap 30 allows electrical connection to the external sensing circuitry (not shown), as illustrated by a wire lead 32. Additionally, an adhesive layers 33 and 34 are typically deposited between the electrodes, 18 and 26, and the corresponding substrate surface, 14 and 16, respectively, to enhance adherence of the electrodes to the substrate surface. Finally, depending upon the application, a sorbent selective film (not shown) may cover the sensing surface 16.
During operation of the sensor 10, a variable frequency oscillator (not shown) is electrically connected to the reference and sensing electrode taps, 22 and 30, and the sensing surface 16 is inserted into an environment, which may be either a gas or a liquid, while the reference surface 14 remains exposed to air. The environment contains a measurand, which is a specific property of the environment that is being sensed by the sensor, such as, for example the concentration of a certain substance within a gas or liquid. Thus, when the sensing surface 16 is inserted into an environment, the sensing surface is exposed to a specific measurand contained within the environment. Should the sensing surface be covered by a sorbent film, the sorbent film also is immersed in the environment. The oscillator applies a varying voltage to the electrodes, 18 and 26, which then generate acoustic waves within the substrate 12. Such a mode of operation is referred to as Thickness Field Excitation (TFE). Before exposing the sensing surface 16 to the measurand the sensor 10 is calibrated by varying the oscillator frequency to resonate the sensor 10. The resonance frequency is detected and stored in a conventional device or circuit (not shown). After calibration, the sensing surface is inserted into the environment being monitored. The effect of mechanical loading properties of the measurand, such as mass, density and viscoelasticity, upon the sensing surface 16 causes the resonant frequency of the sensor to shift. The shift in resonant frequency can be calibrated to be indicative of the magnitude of a specific mechanical loading property of the measurand.
Alternate embodiments of the QCM sensor 10 having different sensing electrodes are illustrated in FIGS. 3 through 5. FIG. 3 illustrates small electrode geometry with a very small circular sensing electrode 35. A typical diameter for the sensing electrode 35 would be about 0.8 mm. In FIG. 4, a closed ring geometry sensing electrode 36 that has an aperture formed through the center of the electrode disc is shown, while FIG. 5 illustrates an open ring sensing electrode 38. The open ring electrode 38 is very similar to the closed ring electrode 36, except that the open ring electrode 38 has a slot 40 extending through the ring that corresponds to the tap region of the reference electrode. Both the closed and open ring electrodes 36 and 38 have an outside diameter of approximately 13 mm and an inside diameter of approximately 11 mm. All of the sensors shown in FIGS. 4 through 5 have a reference surface configuration that is similar to the sensor 10 shown in FIG. 1.
The use of conventional QCM sensors, such as the one shown in FIGS. 1 and 2, is limited to applications where measurand electrical properties such as permittivity and conductivity change in addition to changes in the mechanical properties listed above. In many applications, the measurement of changes in the electrical properties is critical. However, with conventional QCM sensors, such as the one shown in FIGS. 1 and 2, the sensing electrode 26 that contacts the measurand is the same size or larger than the reference electrode 18 that contacts air. Because of its size, the sensing electrode 26 shields most of the TSM electric field, preventing the penetration of the field into the measurand. Thus, a conventional QCM sensor has minimal sensitivity to changes in electrical properties of the measurand. The modified sensing electrode geometries shown in FIGS. 3 through 5 reduce the size of the sensing electrode. As a result, a small shift of the resonant frequency of the modified QCM sensors may be detected as the electrical properties of the measurand changes. However, it is desirable to provide a piezoelectric sensor with greater sensitivity to the mechanical and electrical properties of the measurand.